High performance stepper motor

ABSTRACT

A high performance stepper motor comprises a stator having a plurality of pole positions. Each of the pole positions includes a plurality of stator elements or teeth on opposite sides of a stator-to-stator air gap with nonmagnetic material located in the spaces between the extremities of the teeth. Energization means in the form of a plurality of windings adapted to be connected to a multiphased drive source are associated with the stator means at the various pole positions respectively. Motive means comprising a rotor or a slider includes a plurality of motive elements which comprise teeth or slugs. In order to achieve low inertia and a high force-to-mass ratio, the motive means is located within and confined to the air gap of the stator. The motive elements comprise magnetic material and have extremities juxtaposed to the extremities of the stator elements on both sides of the air gap so as to establish flux paths transverse to the air gap and movement of the motive means relative to the stator means with the resulting force acting on the motive elements at both sides of the air gap. In order to further achieve low inertia and a high force-to-mass ratio of the passive means, the motive means comprises discontinuities in the magnetic material thereof including nonmagnetic material between the extremities of the motive elements so as to reduce flux leakage in the direction of movement of the passive means through the air gap while simultaneously reducing the mass of the motive means. By utilizing a passive means of this type, the energization means may be limited to one side of the air gap to achieve manufacturing economies and topological advantages while at the same time achieving high performance.

This is a divisional, of application Ser. No. 809,646, filed June 24,1977, now U.S. Pat. No. 4,198,582 issued Apr. 15, 1980.

BACKGROUND OF THE INVENTION

This invention relates to high preformance stepper motors of the linearor rotary type which may be used in various applications including theelectronic typewriter.

Variable reluctance stepper motors have been conveniently used asincremental motion transducers. However, for applications requiring ahigh force-to-mass ratio, such stepper motors have not, in someinstances, been used because the force developed per unit of moving massis not as high as for certain DC motors including both commutator typerotary DC motors and voice-coil linear DC motors.

Fredrickson U.S. Pat. No. 3,292,065 discloses a variable reluctancestepper motor of the linear type. In the embodiment of Fredrickson whichachieves the highest force-to-mass ratio, the stator means is excited atthe various pole positions on both sides of the longitudinally extendedstator-to-stator air gap, i.e., double-sided excitation. However, theforce-to-mass ratio of Fredrickson suffers for two reasons. First, theFredrickson structure permits flux leakage longitudinally through theslider. In this connection, it will be noted that the nonmagneticdiscontinuities between the teeth of the slider are relatively shallow,i.e., these discontinuities do not extend perpendicular or transverse tothe direction of movement of the slider a distance substantially greaterthan the space between the teeth. Moreover, there are no additionaldiscontinuities of nonmagnetic material in the slider. Accordingly, arather unlimited longitudinal flux path through the magnetic material ofthe slider may be established for flux leakage which reduces theforce-to-mass ratio for the slider of the motor. Second, the slider ofFredrickson extends outwardly beyond the ends of the stator so that asubstantial portion of the slider is not generating force to drive theslider. Moreover, the slider portion extending beyond the air gapcreates end effects which are detrimental to the drive. It will also benoted that the highest force-to-mass ratio embodiments disclosed in theFredrickson patent require stator means with winding or excitation meanson both sides of the longitudinally extending air gap which receives theslider.

Chai U.S. Pat. No. 3,867,676 also discloses a variable reluctancestepper motor of the linear type with double-sided excitation. Chaidemonstrates no apparent concern for longitudinal flux leakage or itsadverse effects on force-to-mass ratio. Significantly, the minimumthickness of the Chai slider which provides a longitudinal flux leakagepath is always substantial relative to the maximum thickness at theextremities of the teeth, e.g., the minimum thickness is at least 25% ofthe maximum thickness. There is no discussion of an effort to minimizelongitudinal flux leakage nor is there any suggestion in thespecification that such longitudinal flux leakage has been minimized. InFIG. 9 where Chai achieves the minimum longitudinal flux leakage sincethe thickness of the slider is substantially less than the maximumthickness, the force-to-mass ratio is particularly small. This smallforce-to-mass ratio is the result of a single tooth per stator pole anda slider which always includes a substantial portion which extendsbeyond the stator structure and thereby produces no force.

Similarly, Schreiber et al U.S. Pat. No. 3,162,796 also discloses avariable reluctance stepper motor of the linear type but demonstrates nointerest in achieving a high force-to-mass ratio. In all of theSchreiber et al embodiments, there is a single tooth per stator pole andthe slider extends beyond the stator structure so as to produce a smallforce-to-mass ratio. In almost all of the Schreiber et al embodiments,there is no stator structure on the interior of the cylindrical slider,and the minimum thickness of the slider is almost as great as themaximum thickness so as to permit return of the flux longitudinallythrough the slider which necessarily reduces the force-to-mass ratio.The embodiment of FIGS. 14 and 15 does disclose the use of interiorstator structure which permits a "reduction in weight" of the slideralthough there is no suggestion that the force-to-mass ratio isincreased and the shape of the teeth which preclude any effectivegeneration of force by the exterior stator structure in FIG. 14 and theinterior stator structure in FIG. 14 suggest a low force-to-mass ratio.In connection with FIG. 15, there is the suggestion that the portion ofthe slider between the teeth may even comprise a nonmagnetizablematerial. However, there is no suggestion that the nonmagnetizablematerial is chosen for purposes of limiting longitudinal flux leakageand the suggestion that the material be "austenitic boron steel"precludes a further reduction in weight.

An article entitled Characteristics of a Synchronous Inductor Motor,Snowdon and Madsen, Trans, AIEE (Applications in Industry) vol. 8, pp.1-5, March 1962, discloses a stepper motor having a rotor which isconfined to the air gap of the stator. However, the force-to-mass ratiois relatively small since the rotor acts as a longitudinal flux returnpath to a single-sided stator. In order to provide this longitudinalflux return path, the minimum thickness of the rotor between the teethof the rotor is substantial relative to the maximum thickness of therotor at the teeth. An article entitled A Self-Oscillating InductionMotor for Shuttle Propulsion, Laithwaite and Lawrenson, Proc. IEE. vol.104, part A, No. 14, April 1957, suggests that the rotor of Snowdon andMadsen might be unwound. However, the resulting slider would still haveto provide a longitudinal flux return path for a single-sided slider.Therefore, even if the slider were shortened as disclosed in an articleentitled Linear Induction Motors, Laithwaite, IEE, paper No. 2433 u,December 1957, the slider would still have a relatively lowforce-to-mass ratio and while this force-to-mass ratio might beincreased by utilizing the double sided stator disclosed in LinearInduction Motors, the configuration of the slider with its longitudinalflux leakage still severely limits the force-to-mass ratio.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a stepper motor with a highforce-to-mass ratio.

It is a further object of this invention to provide a stepper motorhaving geometric advantages in combination with a high force-to-massratio.

It is also an object of this invention to provide a stepper motor at arelatively low cost while still preserving the high force-to-mass ratio.

In accordance with these and other objects of the invention, a highperformance stepper motor comprises stator means including a pluralityof pole positions on only one side of the air gap with each of the polepositions including a plurality of magnetic stator elements on oppositesides of a stator-to-stator air gap with nonmagnetic material located inthe spaces between the extremities of the magnetic stator elements onopposite sides of the air gap. Winding means are associated with themagnetic stator elements at the various pole positions and are adaptedto be excited so as to generate flux paths transverse to the air gap.

Passive motive means are located in the air gap. The passive motivemeans comprises a plurality of passive magnetic motive elementsseparated by nonmagnetic material. The motive elements have extremitiesjuxtaposed to the extremities of the activating elements so as to closethe transverse flux paths through the motive elements. In accordancewith this invention, the motive means and the motive elements are alwaysconfined to the air gap with magnetic force acting on both sides of theair gap, and the motive means further comprises nonmagneticdiscontinuities including nonmagnetic material so as to eliminate anysubstantial longitudinal flux leakage.

In further accordance with this invention, the activating meanscomprises discontinuities in the magnetic material including nonmagneticmaterial between the extremities of the passive elements so as to reducethe flux leakage path in the direction of movement of the passive meansto less than 25% of the flux through the transverse flux paths. In aparticularly preferred embodiment of the invention, discontinuities inthe magnetic material decrease the flux leakage to less than 10% of theflux through the transverse flux paths. At the same time, thediscontinuities comprising nonmagnetic material reduce the mass of themotive means so as to maximize the force-to-mass ratio of the motivemeans. In preferred embodiments of the invention, the longitudinal fluxleakage of the motive means is minimized and the force-to-mass ratio ismaximized by providing discontinuities which limit the minimum thicknessof the motive means to less than 25% of the maximum thickness at theextremities of the motive elements and preferably less than 15%.

The discontinuities in the motive means may be formed by providing themotive means with a plurality of teeth on opposite sides thereof so asto form elements juxtaposed to the stator elements. Holes may beprovided in the motive means to achieve additional discontinuities. Inthe alternative, the motive means may comprise discrete elementsseparated by discrete elements of nonmagnetic material therebetween.

In accordance with another important aspect of the invention, the motivemeans may be in the form of a slider as in the case of a linear motor ora rotor in the case of a rotary motor. Where the motive means comprisesa slider, the slider is short relative to the length of the stator.Where the motive means comprises a rotor, the motive means may besubstantially planar when used in an axial air gap stepper motor or therotor may be cup-like for use in a radial air gap stepper motor.

In accordance with another important aspect of the invention, thewinding means associated with the activating means is limited to asingle side of the air gap. This single-sided energization may beembodied in linear stepper motors as well as rotary stepper motorsgiving important geometric advantages, i.e., elimination of the windingson one side of the air gap affords greater freedom in mounting themotors and attaching loads to the motive means. When single-sidedenergization is employed, it is particularly important to achieve lowlongitudinal flux leakage through the motive means so as to maximize theforce-to-mass ratio. It is also important to minimize longitudinal fluxleakage so as to assure that the forces on the motive means at each sideof the air gap are substantially equal or balanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a linear stepper motor embodying theinvention;

FIG. 2 is an enlarged, partially schematic view of a slider in the airgap of the stepper motor shown in FIG. 1 at one pole position;

FIG. 3 is a partial, slightly enlarged elevational view of a linearstepper motor embodying the invention;

FIG. 4 is an enlarged view of the slider in the linear motor of FIG. 3;

FIG. 5 is an elevational view of an alternative slider incorporating theprinciples of this invention;

FIG. 6 is an elevational view of another slider incorporating theprinciples of this invention;

FIG. 7 is an exploded view of an axial air gap rotary motorincorporating the principles of this invention;

FIG. 8 is a plan view of the rotor in FIG. 7;

FIG. 9 is a sectional view taken through line 9--9 of FIG. 8;

FIG. 10 is a sectional view along the axis of a radial air gap rotarymotor incorporating the principles of this invention;

FIG. 11 is a sectional view of the motor of FIG. 10 taken along line11--11 of FIG. 10; and

FIG. 12 is a partial view of the rotor cup of the motor of FIG. 10 takenat line 12--12 of FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, a variable reluctance linear stepper motor isdisclosed comprising a stator 10. The stator 10 comprises an activeportion 12 and an inactive portion 14 which form stator-to-stator airgap 16 between a plurality of pole positions 18₁, 18₂, 18₃, and 18₄.Energization means in the form of windings 20₁, 20₂, 20₃, and 20₄ areprovided at each of the pole positions 18₁ -18₄ on the active portion 12of the stator 10. By utilizing single-sided excitation, the structure onone side of the air gap may be substantially eliminated so as to providesubstantial geometric advantages, manufacturing economies, and weightreduction. However, it becomes particularly important to limitlongitudinal flux leakage through the slider 22 to achieve a maximum,balanced force on the passive motive means.

The passive motive means in the form of a slider 22 is located in theair gap 16 for linear movement past the various pole positions 18₁₋₄. Asbest shown in FIG. 2, the slider 22 comprises a plurality of passivemagnetic motive elements or teeth 24 which extend longitudinally throughthe air gap 16 and along the slider 22. The teeth 24 cooperate withstator elements or teeth 26 of the stator 10 on both sides of the airgap 16 at each of the pole positions 18₁₋₄ so as to close transversemagnetic flux paths across the gap 16.

In accordance with this invention as shown in FIG. 2, the slider 22comprises substantial nonmagnetic discontinuities in the form ofnonmagnetic material 28 between the teeth 24 which substantiallyeliminates any longitudinal flux leakage through the slider 22 while atthe same time reducing the weight of the slider thereby maximizing theforce-to-mass ratio of the motor. In further accordance with thisinvention as shown in FIG. 1, the slider 22 is short relative to theoverall length of the air gap 16 so as to assure that the slider 22 andall of the teeth 24 are always confined to the air gap 16 so as tofurther maximize the force and eliminate any end effects on the slider22. This assures that force is generated with respect to substantiallyall of the slider teeth 24. In FIG. 2, the slider 22 is schematicallyshown as comprising nothing more than moving teeth 24 separated bynonmagnetic material 28 comprising air gaps extending completely acrossthe slider 22. In actuality, the teeth must be structurallyinterconnected although the weight of the interconnecting structure ispreferably minimized so as to achieve a high force-to-mass ratio.Reference will no be made to FIGS. 3 and 4 where the slider 22 is shownas comprising discrete and solid nonmagnetic inserts and the windings20₁, 20₂, 20₃, and 20₄ are shown in more detail.

As shown in FIG. 3, each of the two windings 20₁ are connected in seriesas are each of the two windings 20₂. A multiphased drive or source isconnected to the windings 20₁ and 20₂ such that one phase P₁ isconnected to the windings 20₁ and another phase P₂ is connected to thewindings 20₂. Similarly, other drive phases are connected to partiallyshown windings 20₃ and 20₄ such that the windings 20₁₋₄ are sequentiallyexcited so as to sequentially energize the pole positions 18₁₋₄.

As shown in FIG. 3, the teeth 24 at the pole position 18₃ are preciselyjuxtaposed to the teeth 26 at that same pole position while the teeth 24at the remaining pole positions are staggered with respect to the teeth26. This is a result of simultaneous energization of the windings 20₂,20₃, and 20₄ during one of eight phases or steps of excitation. The fluxpaths F_(p) extending longitudinally through the stator 10 andtransversely across the slider 22 are shown in FIG. 1 for simultaneousenergization of the windings 20₂, 20₃ and 20₄. During the next or secondphase of excitation, the windings 20₃ and 20₄ are energized so as tomove the slider 22 a distance equal to 1/8th of the pitch p of the teeth26. In the next phase of excitation, the windings 20₃, 20₄ and 20₁ aresimultaneously energized so as to produce another step movementrepresenting 1/8th of the pitch p of the teeth 26 with precise alignmentbetween the teeth 24 and the teeth 26 at the poled position 18₄. Thisincremental movement of the slider 22 continues in accordance withprinciples well known in the art. In the alternative, four phases ofexcitation may be utilized so as to achieve a stepped distance of 1/4ththe pitch p between the leading edges or trailing edges of two adjacentteeth 26 with each of the four following winding excitations.

As stated previously, the longitudinal flux leakage through the slider22, i.e., the longitudinal flux leakage in the direction of longitudinalmotion through the air gap 16, is minimized so as to achieve a highforce-to-mass ratio. This minimum longitudinal stator or flux leakage isachieved by providing magnetically distinct teeth 24 with substantialnonmagnetic material therebetween so as to substantially if notcompletely eliminate the longitudinal flux through the slider 22. In theother embodiments of the invention which will be described herein, thelongitudinal flux is less than 25% of the transverse flux and preferablyless than 10%. In the embodiment of FIGS. 1-4 the teeth 24 may beadhesively secured to the nonmagnetic material inserts 28. It has beenfound that the nonmagnetic material may comprise glass filled epoxy orceramic and a particulary suitable adhesive is an epoxy resin. As shownin FIG. 2, transverse flux is depicted by an arrow F_(t) andlongitudinal flux is indicated by an arrow F_(L). Since the longitudinalflux is substantially eliminated in this embodiment, the arrow F_(L) hasbeen shown in phantom. By eliminating the longitudinal flux leakage, allof the flux generated by the stator 10 produces useful force on theslider. Furthermore, the absence of longitudinal flux leakage means thatthe forces applied to the slider 22 at each side of the air gap arebalanced so as to eliminate undesirable attractive forces between theslider 22 and one side of the stator 10.

In further accordance with the objective of achieving a highforce-to-mass ratio for the slider 22 which is achieved by minimizingthe weight of the material 28, each pole position 18₁₋₄ comprises aplurality of teeth 26 so as to maximize the force generated at each poleposition. Eight such teeth 26 have been shown although any number ofteeth per pole may be used, but the teeth are preferably thin tomaximize the force and minimize eddy currents. Of course, the number ofteeth 24 in the slider 22 would be adjusted accordingly.

Reference will now be made to FIG. 5 wherein another slider 32constructed in accordance with the principles of this invention isdisclosed. As shown therein, teeth or actuated elements 34 extendoutwardly from a central support section 36 which adds only slightly tothe weight and creates only a very small path for undesirablelongitudinal flux leakage. The nonmagnetic material between the teeth 34which is formed by air spaces 38 extend perpendicular to the directionof longitudinal movement through the air gap 16 shown in FIG. 3 adistance substantially greater than the thickness of the support section36, i.e., the minimum thickness of the slider.

In accordance with this invention, the teeth 34 of the slider 32 are ofsufficient height such that the minimum thickness t_(min) at the supportsection 36 is less than 25% of the maximum thickness t_(max) andpreferably less than 15%.

As indicated by the arrow F_(L), there is some longitudinal flux throughthe support segment 36 of the passive slider shown in FIG. 5. However,the longitudinal flux F_(L) is still less than 25% of the transverseflux F_(t) and preferably less than 10%. As a result, a highforce-to-mass ratio is achieved. It will of course be understood thatthe slider 32 is intended for use in the stator of FIGS. 1 and 3 and theslider is therefore short relative to the overall length of the stator10 and the teeth 34 correspond in number and spacing to the teeth 24.

FIG. 6 discloses yet another passive slider 42 including a plurality ofpassive elements or teeth 44 located on opposite sides of a supportsegment 46 which is also adapted for use with the stator of FIGS. 1 and3. In order to limit the longitudinal flux F_(L) through the supportsegment 46 and maximize the transverse flux F_(t), the support segment46 includes a number of openings 50 of various sizes and configurationswhich diminish the longitudinal flux path and further serve to reducethe mass of the slider shown in FIG. 6. As a result, the force-to-massratio of the linear motor is greatly increased.

Although the slider of FIG. 6 differs somewhat in appearance from thatof FIG. 5, it may also be used with the stator of FIG. 1 and thecritical relationships discussed with respect to the slider of FIG. 3are applicable. For example, the effective minimum thickness t_(min) 1+t_(min) 2 is still small relative to the maximum thickness t_(max),i.e., less than 25% and preferably less than 15%.

In the embodiment of FIG. 6 as well as the embodiment of FIG. 3, thelongitudinal flux leakage F_(L) has been substantially reduced relativeto the transverse flux F_(t). More specifically, the longitudinal fluxis less than 25% of the transverse flux and preferably less than 10%. Inthis embodiment, it will be understood that there is some longitudinalflux leakage around the openings 50 since the transverse flux path F_(T)requires it. Accordingly, t_(min) 1 and t_(min) 2 must be sufficient toaccomodate the transverse flux, and preferably t_(min) 1 and t_(min) 2are greater than twice the tooth width t_(w).

As utilized herein, the maximum thickness or maximum effective thicknessmeans the thickness of the slider or the support segment generallytransverse to the air gap 16. It will of course be understood that thesliders of FIGS. 1-6 do have thickness in a direction perpendicular tothe direction of transverse and longitudinal flux, i.e., in a directionlooking into the air gap 16 of FIGS. 1 and 3. In fact, this dimensionwhich shall be referred to as the width may be substantially greaterthan the maximum thickness of the slider in a direction generallyparallel to the transverse flux. Furthermore, it will be appreciatedthat the sliders 22, 32 and 42 as well as the stator 10 may comprise aplurality of laminations along the width of the slider so as to reduceeddy currents in accordance with the practice well known in the art.

Reference will now be made to FIGS. 7-9 wherein an axial air gapvariable reluctance rotary stepper motor is disclosed which incorporatesthe principles of this invention. More particularly, the motor shown inFIG. 7 comprises a stator 110 including an active portion 112 and aninactive portion 114. In other words, the stator 110 is excited on onlyone side of an air gap 116 which is enlarged as a result of the explodedview of FIG. 5. The stator 110 forms a plurality of pole positions 118₁,118₂, 118₃, and 118₄. Windings 120₁, 120₂, 120₃, and 120₄ are wrappedaround axially extending portions of the pole structure at each of thepole positions 118₁₋₄.

The excited or active portion 112 of the stator 110 comprisesself-supporting magnetic material which forms a plurality of activeelements or teeth 126 at each pole position which are juxtaposed acrossthe air gap 116 to a plurality of stator teeth 130 which comprise slugswhich are mounted in a support member 132 of the unexcited portion 114of the stator 110. It will be understood that the teeth 130 of thevarious pole positions 118₁₋₄ of the unexcited portion 114 are preciselyaligned with the teeth 126 at the various pole positions 118₁₋₄ on theexcited portion 112.

In further accordance with the principles of this invention, passivemotive means in the form of a rotor 122 comprises a nonmagnetic supportstructure 128 which supports and magnetically separates a plurality ofdiscrete magnetic elements or slugs 124. As shown in FIG. 9, the slugs124 extend completely through the nonmagnetic material 128 of the rotor122. As shown in FIG. 8, the slugs 124 are evenly distributed on radiallines around the rotor 122. For simplicity, not all of the slugs 124have been shown. It will however be understood that they are evenlydistributed over 360°.

The windings 120₁₋₄ are selectively energized in the same way as thewindings 20₁₋₄. More specifically, the windings may be energized asdiscussed in the foregoing. With such energization, the rotor 122 willadvance 1/8th of the pitch between the teeth 126 so as to achieveprecise alignment between the slugs 124 and the teeth 126 and 130 one ofthe pole positions 118₁₋₄ every other phase of energization.

Although it has not been shown, it will be understood that a rotatablesupport member would extend through the opening 130 of the rotor 122 andinto a bearing support opening 132 of the unexcited or inactive portion114 and a similar support opening in the excited or active portion 112of the stator 110.

In the embodiment of FIGS. 7-9, the longitudinal flux leakage in adirection parallel to or coincident with the rotational motion of therotor 122, is eliminated. This is accomplished by eliminating anymagnetic path between the slugs 124. It will be understood that variousdesign features of the linear slider shown in FIGS. 5 and 6 might beincorporated in the rotor 122 while still achieving the all-importantlow longitudinal flux leakage so as to promote a high force-to-massratio for the rotor 122. For example, the rotor 122 which is analogousto the slider 22 may be formed in the manner of the slider shown in FIG.5 where the teeth are contiguous with a magnetic support section and theoverall thickness t_(min) of that magnetic support section issufficiently small relative to the overall or maximum thickness t_(max)of the rotor so as to reduce the longitudinal flux leakage to less than25% of the transverse flux through the air gap 116 and preferably lessthan 10%. Similarly, the design of the slider 42 shown in FIG. 6 mightbe incorporated in the rotor 122 so as to achieve reduced longitudinalflux leakage and a high force-to-mass ratio.

Reference will no be made to FIGS. 10-12 which disclose a radial air gapvariable reluctance rotary stepper motor which also incorporates theprinciples of this invention. Once again, the stator 210 is excited on asingle side. More particularly, the stator 210 comprises an activeportion 212 which is wrapped with windings 220₁₋₄ at the various polepositions 218₁₋₄ and an inactive portion 214. A radial air gap 216 isformed between active elements or teeth 224 on opposite sides of the airgap 216.

As best shown in FIG. 11, the excited portion 212 of the stator 210comprises a plurality of active elements or teeth 226 which areseparated by nonmagnetic material in the form of air gaps at each of thepole positions 218₁₋₄. In a similar way, the teeth 226 are formed in theunexcited or inactive portion 214 of the stator 210.

In accordance with this invention, a rotor 222 which extends into theaxial air gap 216 comprises a plurality of discrete passive elements orslugs 224 which are carried by and separated from one another bynonmagnetic material 228 which forms a rotor cup 230. As shown in FIGS.9 and 10, the slugs 224 extend through the rotor cup 230 so as to be inclose proximity to the active elements or teeth 226 of the stator 210.

As shown in FIGS. 10 and 11, the rotor cup 230 is supported by a shaft232 which extends through the excited portion 212 of the stator 210 anda housing 234. An annular portion 236 extends outwardly from the cup 230and may be attached to a suitable load.

As in the previous embodiments, the radial air gap embodiment of FIGS.10-12 may utilize a rotor analogous to the slider of FIGS. 5 and 6. Moreparticularly, the rotor cup may be integrally formed from the magneticrotor material with the support portion for the teeth of the rotor cupminimized in thickness so as to substantially reduce if not eliminatethe longitudinal flux.

In the various embodiments described in the foregoing, the stator hasbeen excited on only one side. With such single-sided excitation, it isparticularly important to minimize longitudinal flux leakage through theslider or rotor and confine all longitudinal flux leakage to the stator.This assures the generation of the maximum force while enjoyingsubstantial topological advantages, i.e., the windings need only belocated on one side of the air gap. At the same time, manufacturingeconomies and motor weight reductions may be achieved. However, theprinciples of this invention are also applicable to a stepper motorwhere both sides of the stator are energized by windings.

Referring now to FIGS. 1 and 3, a multiphase drive associated with thewindings 20₁₋₄ has been described. Moreover, the multiphase drive hasbeen described as providing the selective excitation of the windings infour different phases so as to permit the slider 22 to advance 1/4th ofthe pitch of the teeth for each particular phase. As shown in FIGS. 1and 3, the windings 20₁ have been shown in series as have the windings20₂. This series configuration is preferred because this minimizes theAC component of flux that passes through a give pole when the slider isnot in the vicinity of that pole. However, the windings 20₁₋₄ need notbe connected in series. It is also generally considered preferable toexcite the pole so that adjacent poles have opposite polarization. Inmost cases where rapid motion is desired, the amount of flux passingthrough the poles will vary as exciting current varies but the polarityof the magnetization will not change. In some cases, it may even bedesirable to provide some portion of the magnetization from a separateDC bias winding of from a permanent magnet. It will of course beappreciated that any intermediate step may be achieved by the properchoice of phase currents.

As shown in all embodiments of the invention, the motors have four phasewindings. It is however possible to change the number of phases. Forexample, the number of phases may be reduced from four to three.Further, the number of teeth or active elements at each pole positionmay be varied, i.e., they may be increased from the eight shown inFIG. 1. Also, the number of pole positions may be varied.

In all of the embodiments of the invention, the spacing of the teeth orelements of the activating means or stator and the passive means orslider or rotor have been maintained as equal. This is not necessary.For example, every other tooth on the slider or rotor could be removedor the number of teeth per unit length could be slightly different forthe stator and the slider or rotor. Also, the width of the teeth t_(w)relative to the pitch p may be varied. For example, the teeth 24 and thenonmagnetic inserts 28 are shown as having equal width in FIGS. 3 and 4.It will be understood that this relationship may be varied and, in fact,it is preferred that the width of the teeth 24 be approximately 33% ofthe width of the inserts 28.

For various details of linear or rotary stepper motors which have notbeen set forth herein, reference is made to "Theory and Application ofStep Motors," edited by Benjamin Kuo, West Publishing Co., 1974. Thispublication will, for example, describe in detail the structure andprinciples of stepper motors of the rotary and linear type.

As used herein the phrase stepper motor embraces variable reluctancemotors which, because of their magnetic structure, are capable ofoperating in a mode which produces movement in discrete steps. However,the phrase is also intended to cover motors of this type which areoperated in a mode producing continuous positioning.

It is also possible to achieve induction motor operation by suitablyimbedding the teeth of the passive means in a conducting material. Thiscould aid in starting or synchronizing or could provide the dominantforce mechanism.

Although specific embodiments of the invention have been shown anddescribed, it will be understood that other embodiments andmodifications which will occur to those of ordinary skill in the artfall within the true spirit and scope of the invention as set forth inthe appended claims.

What is claimed is:
 1. A high performance rotary stepper motorcomprising:stator means having an active and passive portion, eachcomprising a plurality of pole positions, each of said pole positionsincluding a plurality of magnetic stator elements on opposite sides ofan axial air gap with nonmagnetic material located in the spaces betweenthe extremities of said magnetic stator elements on each of saidopposite sides of said air gap; winding means associated with saidactive portion at said pole positions on only one side of said air gapfor generating flux paths transverse to said air gap; a rotor locatedwithin said air gap comprising a plurality of passive magnetic motiveelements and nonmagnetic discontinuities including nonmagnetic materialseparating said motive elements, said motive elements having extremitiesjuxtaposed to the extremities of said stator elements so as to closesaid transverse flux paths through said motive elements, said motivemeans and said motive elements having configurations such that all ofsaid motive elements are always confined to said air gap with magneticforce acting on said magnetic motive elements at both sides of said airgap, said rotor further comprising nonmagnetic discontinuities includingsaid nonmagnetic material so as to eliminate any substantiallongitudinal flux leakage.
 2. The stepper motor of claim 1 wherein saidrotor comprises a substantially planar rotor adapted for rotary motionthrough said air gap.
 3. The stepper motor of claim 2 wherein saidmotive elements comprise a plurality of discrete magnetic elements andsaid rotor further comprises a disc comprising nonmagnetic material inwhich said discrete magnetic elements are mounted.
 4. The stepper motorof claim 1 wherein said active and passive portions are separated bysaid axial air gap and having no flux path between active and passiveportion except through said motor.